Trunk piston



March 6, 1956 w. F. PETERSON TRUNK PISTON Filed Sept. 16, 1950 w m m w.

FIG. I.

WILLIAM F, PETERSON ATTORNEYS.

United States Patent TRUNK PISTON William F. Peterson, Kirkwood,Mo.,assignor, by mesne assignments, to Sterling Aluminum Products, Inc.,St. Louis, Mo., a corporation of Missouri Application September 16,1950, Serial No. 185,261

'8 Claims. (Cl. 30913) This invention pertainsto trunk pistons such asare used in internal combustion engines and in particular to suchpistons constructed of light metals, such as aluminum and magnesium andtheir alloys, which are of relatively high thermal expansion. The methodof making the trunk piston is claimed in application Serial No. 553,083,filed December 14, 1 955.

A trunk piston of the type described comprises a head, a skirt, andwrist pin bosses on the inside of the skirt. The head is grooved toreceive the piston rings and is of smaller diameter than the skirt,which provides the guiding cross-head for the piston. The skirtisseparated from the head in one or both regions of the thrust faces bycircumferential slots. The skirt may be of circular contour but isusually made oval with the major diameter in the regions of the thrustfaces and with the minor diameter in the regions of the bosses. Somepistons have vertical slots.

The action of such a piston in an internal combustion engine is asfollows: When heat is applied to the head, such heat is transmitted tothe skirt in the regions of the bosses because of separation of theskirt from the head in the region or regions of a thrust face or faces.That causes the skirt to expand along the wrist pin boss axis where theoutside of the skirt is relieved, resulting in drawing in of the skirtalong the thrustaxis. Such an action has been well-known for years.Where, however, the piston is of a metal such as aluminum, magnesium,and their alloys, of high thermal expansion, difficulty is experiencedin having the piston fit closely to the cylinder walls at differenttimes without seizing or slapping; therefore, resort was had to strutsand other control elements of a metal of low thermal expansion such assteel.

One of the objects is to provide a light metal piston of novelstructure, having a control element of a lower coeflicient of expansionthan the skirt applied in such a manner as to control expansion of theskirt diameter.

' Other objects will be apparent to those skilled in the art in thelight of the following specification and accompanying drawing.

Referring now to the drawings:

Figure 1 is a view in side elevation partly cut away, showing oneembodiment of this invention;

Figure 2 is a sectional view taken along the line 2-2 of Figure 1;

Figure 3 is a bottom plan view;

Figure 4 is a sectional view taken along the line 4-4 of Figure 1; and

Figure 5 is a perspective view of a U-shaped insert for use with thisinvention. 5

Referring now to the accompanying drawing, 1 designates the skirt and 2the head of a piston of the character described; that is, one of a metalsuch as aluminum or magnesium or their alloys of high thermal expansion.The skirt is provided with thrust faces 3, and with wrist pin bosses 4,connected to the head by strengthening ribs '40, and the head isprovided with the usual piston packing ring grooves 14. The skirt may becircumferentially separated from the head in one or both regions of thethrust faces by a slot or slots 11, and one of these thrust faces may beprovided with a vertical slot 12 extending downwardly from suchseparation. The skirt is thus attached to the head in the regions of thebosses, but separated from the head in one or both regions of the thrustfaces.

The control element as shown in Fig. 5 is made of a metal of low thermalexpansion such as steel. In the particular embodiment described, thecontrol element comprisesa circumferential part 5, and side parts 6,which latter may be straight or of curved shape and which may extendinwardly at their ends.

Referring first to the top of the skirt, a control element 5, 6, ispositioned at the top of and inside of the skirt with its side partsrigidly anchored against movement in the regions of the bosses andspecifically in the vertical strengthening ribs 40, and with itsconnecting circumferential part extending circumferentially along andagainst a separated thrust face. The ends of the side parts may alsobear against the thrust face opposite to that of the circumferentialpart, by the provision of bosses 10. The inside face of thecircumferential part of the control element is exposed and is simplysupported against shifting by ribs 15 and 16. The rib 15, may be of ashallow dimension radially, as shown in Figs. 2 and 4, and may extendnot only along the circumferential part of the control element, but alsoalong the side parts thereof to the vertical ribs 40. Likewise, the rib16, which as shown in Figs. 2 and 4 has a radial dimension to the insideedge of the control element,.also extends along the exposedcircumferential part and the side parts.

In making this piston the control element is placed in a mold, usuallyof the permanent type, in which the piston is cast, and on a suitablepermanent core, which in pistons of this type comprises a three-piececenter core and two side cores, the later containing recesses, formingthe wrist pin bosses 4, while the mold may be provided with cores toform the holes in the bosses. The outer pieces of the center core aswell as the side cores are slotted as usual to receive the controlelement. At this time, of course, all circumferential and verticalslotting such as the slots 11 and 12, are absent as are also the pistonring grooves. After the piston has been cast in the manner described andhas solidified the cores are withdrawn as usual in molding practice bypermanent molds and the mold is opened.

solidification of the casting will cause the piston metal to shrink andcontract faster than the control element. This will result in the sideparts of the control element to be rigidly cast-anchored againstmovement in the regions of the bosses by shrinking thereon of the pistonmetal and the bosses 10 will also shrink on the side parts of thecontrol element. This will place the circumferential part of the controlelement under compression radially with the piston metal along thatcircumferential part placed under tension by shrinking of the pistonmetal on that connecting circumferential part. When, now the skirt isseparated from the head by the formation of the slots 11 above thecircumferential part of the control element, the above-describedcompression and tension will be somewhat relieved and any grippingaxially of the ribs 15 and 16 on the control element will also, uponcooling of the piston, have been relieved.

The piston is now finished by cutting the packing ring grooves 14, bymachining the head to the proper diameter and by machining the skirt tothe desired contour, which as stated above is usually one in which thepiston will be oval, with the minor axis along the wrist pin axis andthe major axis along the thrust face axis. The piston can. however, beground round.

A piston of the character described and made as described can be madewith a very low clearance and it will still maintain a close fit in thecylinder when either hot or cold. As the piston heats up, the head will,of course, expand radially in all directions. Such radial expansion ofthe head will however, only be transmitted to the skirt in the regionsof the bosses. That will tend to move the skirt in along the line of thethrust faces. Such movement is permitted because the steel controlelement is pushed along the wrist pin axis and will pull the steel inalong the thrust face axis and that will be followed by the piston metal(aluminum) outside of the circumferential part 5. The side parts 6, arehowever rigidly anchored in the thrust faces because the metal is castentirely therearouncl. The piston metal does not however surround thecircumferential part 5, but only lies against the outside thereof and,in the particular embodiment shown, against the side thereof. Thecontrol element having its inner portion exposed circumferentially andaxially, said control element at said thrust face and the metalthereover is therefore under radial stress when the piston is cold,which stress is progressively relieved as the piston heats up in theengine so as to maintain the skirt dimension diametrically. The skirt atthe thrust faces will therefore remain accurately dimensioned by thecontrol element.

If desired, the thrust face opposite that at which the circumferentialpart of the control element is positioned, may also be separated fromthe head by a circumferential slot 11, and may be provided with avertical slot 12. That renders the piston somewhat more flexible.

In the embodiment shown in Figs. 1 and 2, there is a second controlelement positioned at the bottom of and inside of the skirt, anchored inthe regions of the bosses and extending circumferentially along andagainst the separated thrust face. Such a control element, like thatshown in Fig. 5, is shown as anchored-between ribs 41 and 42, in theregions of the bosses with the side parts also anchored in bosses andwith the circumferential part 5 between ribs 41 and 42 similar to theribs and 16. In fact, the rib 42 may be omitted as shown in Fig. 3. Sucha control element at the bottom of the skirt may be used with a controlelement at the top of the skirt. It is formed in the same way in thecasting of the piston in a permanent mold on a permanent core.

It will therefore be seen that the invention accomplishes itsobjectives. .A piston is provided which can be closely fitted to acylinder of an internal combustion engine and which will maintain itsdimensions from hot to cold so that there will be no seizing when hotand no slapping when cold.

Having thus described the invention, what is claimed 1. A trunk pistonof a metal of relatively high thermal expansion, comprising, a head, askirt provided with wrist pin bosses and with thrust faces, one of saidthrust faces being circumferentiallyseparated from said head, and acontrol element of a metal of low thermal expansion positioned at thetop of and inside of said skirt, said control element having side partsrigidly anchored against movement in the regions of the bosses andhaving a connecting part extending circumferentially along and againstsaid separated thrust face having its inner portion exposedcircumferentially and axially, said control element at said thust faceand the metal thereover being under radial stress when the piston .iscold, which stress is progressively relieved as the piston heats up inthe engine, adapted to maintain the skirt dimension diametrically.

2. A trunk piston of a metal of relatively high thermal expansion,comprising, a head, a skirt provided with wrist pin bosses and withthrust faces, one of said thrust faces being circumferentially separatedfrom said head, and a control element of a metal of low thermalexpansion positioned at the top of and inside of said skirt and overwhich element the skirtis cast, said control element having side partsrigidly cast-anchored against movement in the regions of the bosses byshrinking thereon of the skirt metal and having a connecting partextending circumferentially along and against said separated thrust facebut having its inner portion exposed circumferentially and axially, saidcontrol element at said thrust face and the metal thereover being underradial stress when the piston is cold, which stress is progressivelyrelieved as the piston heats up in the engine, adapted to maintain theskirt dimension diametrically.

3. A trunk piston of a metal of relatively high thermal expansion,comprising, a head, a skirt provided with wrist pin bosses and withthrust faces, one of said thrust faces being circumferentially separatedfrom said head, and a control element of a metal of low thermalexpansion positioned at the top of and inside of said skirt and overwhich element the skirt is cast, said control element having side partsrigidly cast-anchored against movement in the regions of the bosses byshrinking thereon of the skirt metal metal and having a connecting partextending circumferentiallyalong and against which bears said separatedthrust face by shrinking of the skirt metal radially inwardly on saidconnecting part, said connecting part having its inner portion exposedcircumferentially and axially, said control element at said thrust faceand the metal thereover being under radial stress when the piston iscold, which stress is progressively relieved as the piston heats up inthe engine, adapted to maintain the skirt dimension diametrically.

4. A trunk piston of a metal of relatively high thermal expansion,comprising, a head, a skirt provided with wrist pin bosses and withthrust faces, one of said thrust faces being circumferentially separatedfrom said head, and a control element of a metal of low thermalexpansion p0si tioned at the top of and inside of said skirt, saidcontrol element having side parts rigidly anchored against movement inthe regions of the bosses and in one thrust face, said control elementhaving a connecting part extending circumferentially along and againstthe opposite separated thrust face having its inner portion exposedcircumferentially and axially, said control element at said thrust faceand the metal thereover being under radial stress when the piston iscold, which stress is progressively relieved as the piston heats up inthe engine, adapted to maintain the skirt dimension diametrically.

5. A trunk piston of a metal of relatively high thermal expansion,comprising, a head, a skirt provided with wrist pin bosses and withthrust faces, one of said thrust faces being circumferentially separatedfrom said head, and a control element of a metal of low thermalexpansion positioned at the top of and inside of said skirt, saidcontrol element having side parts rigidly anchored against movement inthe regions of the bosses and having a connecting part extendingcircumferentially along and against said separated thrust face, saidconnecting part bearing axially against a shoulder on the inside of saidthrust face but having its inner portion exposed circumferentially andaxially, said control element at said thrust face and the metalthereover being under radial stress when the piston is cold, whichstress is progressively relieved as the piston heats up in the engine,adapted to maintain the skirt dimension diametrically.

6. A trunk piston of a metal of relatively high thermal expansion,comprising, a head, a skirt provided with wrist pin bosses and withthrust faces, one of said thrust faces being circumferentially separatedfrom said head, and a control element of a metal of low thermalexpansion positioned at the top of and inside of said skirt, saidcontrol element having side parts rigidly anchored against movement inthe regions of the bosses and having a connecting part extendingcircumferentially along and against said separated thrust face, theother thrust face being also circumferentially separated from said head,said .connecting part having its inner portion exposed circumferentiallyand axially, said control element at said thrust face and the metalthereover being under radial stress when the piston. is cold, whichstress is progressively relieved as the piston heats up in the engine,adapted to maintain the skirt dimension diametrically.

7. A trunk piston of a metal of relatively high thermal expansion,comprising, a head, a skirt provided with wrist pin bosses and withthrust faces, one of said thrust faces being circumferentially separatedfrom said head, and a control element of a metal of low thermalexpansion positioned at the top of and inside of said skirt, saidcontrol element having side parts rigidly anchored against movement inthe regions of the bosses and having a connecting part extendingcircumferentially along and against said separated thrust face, theother thrust face being also circumferentially separated from said headand having a vertical slot extending downwardly from such separation,said connecting part having its inner portion exposed circumferentiallyand axially, said control element at said thrust face and the metalthereover being under radial stress when the piston is cold, whichstress is progressively relieved as the piston heats up in the engine,adapted to maintain the skirt dimension diametrically.

8. A trunk piston of a metal of relatively high thermal expansion,comprising, a head, a skirt provided with wrist pin bosses and withthrust faces, one of said thrust faces being circumferentially separatedfrom said head, and a control element of a metal of low thermalexpansion posiseparated thrust face having its inner portion exposedcircumferentially and axially, said control element at said thrust faceand the metal thereover being under radial stress when the piston iscold, which stress is progressively relieved as the piston heats up inthe engine, adapted to maintain the skirt dimension diametrically, and asecond control element positioned at the bottom of and inside of saidskirt, anchored in the regions of the bosses and extendingcircumferentially along and against said separated thrust face buthaving its inner portion exposed circumferentially and axially.

References Cited in the file of this patent UNITED STATES PATENTS1,681,621 Nelson Aug. 21, 1928 1,731,904 Nelson Oct. 15, 1929 1,802,180McCoy Apr. 21, 1931 1,874,925 Diamond Aug. 30, 1932 1,891,419 JardineDec. 20, 1932 2,119,137 Moore May 31, 1938 2,221,535 Berry Nov. 12, 19402,238,086 Bowser Apr. 15, 1941 2,238,087 Fowser et a1 Apr. 15, 19412,240,968 Venner et a1 May 6, 1941 2,426,732 Gates Sept. 2, 19472,551,488 Deming May 1, 1951 FOREIGN PATENTS 136,581 Austria Feb. 26,1934

